Fix path to overlay upper directory in create_tarball.sh

[bedroge]

#618 introduced a new structure where each repo has its own overlay-upper dir, see https://github.com/EESSI/software-layer/blob/2023.06-software.eessi.io/eessi_container.sh#L410 and https://github.com/EESSI/software-layer/blob/2023.06-software.eessi.io/eessi_container.sh#L744. This was not changed in the tarball script yet, leading to errors in #634:

Software directory overlay /tmp/overlay-upper/versions/2023.06 does not exist?!

[eessi-bot]

Instance eessi-bot-mc-aws is configured to build for:

• architectures: x86_64/generic, x86_64/intel/haswell, x86_64/intel/skylake_avx512, x86_64/amd/zen2, x86_64/amd/zen3, aarch64/generic, aarch64/neoverse_n1, aarch64/neoverse_v1
• repositories: eessi.io-2023.06-compat, eessi-hpc.org-2023.06-software, eessi-hpc.org-2023.06-compat, eessi.io-2023.06-software

[eessi-build-deploy-bot-deucalion]

Instance boegel-bot-deucalion is configured to build for:

• architectures: aarch64/a64fx
• repositories: eessi.io-2023.06-software

[eessi-bot]

Instance eessi-bot-mc-azure is configured to build for:

• architectures: x86_64/amd/zen4
• repositories: eessi-hpc.org-2023.06-compat, eessi-hpc.org-2023.06-software, eessi.io-2023.06-software, eessi.io-2023.06-compat

[Neves-P]

LGTM!